The present invention relates to a method for making pure, homogeneous porous glass or ceramic bodies from particulate oxide materials such as fumed metal oxides. The porous bodies may be consolidated into dense, void-free glass or ceramic products.
The concept of making glass products at reduced temperatures by sintering a mass of glass particles is old. A number of methods for producing the mass have been considered, including slip casting, compaction, sedimentation and polymerization or condensation from solutions.
Perhaps the most common methods are the solution methods wherein solutions or suspensions of glass-forming constituents are treated to form a precipitate, polymerization product, or gel which is then dried and sintered to form glass. U.S. Pat. Nos. 3,535,890, 3,678,144 and 4,112,032 describe one such approach to glass-forming wherein silicate solutions or suspensions are gelled, dried and fired to produce glasses. Disadvantages of such approaches include the need for a very prolonged and difficult drying process, and only limited product purity due to the presence of iron and other metallic impurities in the starting material. Drying treatments requiring days, weeks or months for completion are not uncommon.
Higher purity can be attained through the use of starting materials such as alkoxides in the precipitating solutions, as noted in G.B. Pat. No. 2,041,913 disclosing an adaptation of such a process to the manufacture of glass optical waveguides. Again, however, the drying step is difficult and prolonged.
Slip-casting methods have long been used to produce ceramic products from particulate oxide materials, and much of the published literature deals with the technology of producing a satisfactory slip. For example, U.S. Pat. No. 2,942,991 teaches the stabilization of an aqueous casting slip while S. G. Whiteway et al., in "Slip Casting Magnesia", Ceramic Bulletin, 40 (7) pages 432-438 (1961) discuss the advantages of non-aqueous slips. In contrast to the solution methods discussed above, vehicle removal in slip-casting is relatively quick and easy, due in part to the relatively large particle sizes of the suspended oxides. Hence gelation is easily avoided and the problems of gel fragility and cracking are not encountered, although product configuration is somewhat limited.
Attempts to adapt slip casting technology to the handling of high-purity fumed oxides have been made, as reported in U.S. Pat. Nos. 4,042,361 and 4,200,445. The difficulty with fumed oxides arises in part because they are fluffy, high-surface-area materials (surface areas in the range of 25-400 m.sup.2 /g with average particle sizes below 1 .mu.m, typically 0.01-0.1 microns) which are hard to handle and difficult to incorporate into flowable suspensions. In addition, the cast suspensions typically crack on drying in the same manner as the solution-made gels. Thus, the teaching of the aforementioned two patents is to use aqueous suspensions of fumed oxides to prepare a partially sintered, comminuted intermediate (particle sizes of 1-10 .mu.m) for use as a starting material in a conventional slip casting process.
It is proposed in G.B. No. 2,023,571 to deposit oxide glasses of controlled composition by drying and sintering oxides from fluid suspensions on the inside of a heated silica tube. While this technique could be used to produce a composition gradient in the deposited glass, the rate of material deposition is slower than would be desired. Also, adherent, crack-free layers are difficult to obtain, and the method somewhat limits the configuration of the product.
It is therefore a principal object of the present invention to provide an improved method for making glass or ceramic products from fumed oxides by sintering methods.
It is a further object of the invention to provide a method for making a glass product of optical quality by a relatively rapid procedure which facilitates the production of pure, void-free products from a particulate suspension of fumed oxides.
Other objects and advantages of the invention will become apparent from the following description thereof.